The present invention relates to an air conditioning apparatus for a vehicle.
It is generally known that the air conditioning apparatus at cooling operates to radiate heat from coolant compressed by a compressor to the outside through an outdoor heat exchanger for coolant. However, when a vehicle is at a stop where the velocity of cooling wind is remarkably small for the outdoor heat exchanger, the radiation efficiency is deteriorated remarkably.
Therefore, we previously proposed a coolant cycle by which sufficient cooling performance could be effected even if the outdoor heat exchanger is deteriorated in its radiation efficiency (Japanese patent application serial No. 2001-212274). According to the coolant cycle, a water-coolant heat exchanger for exchanging heat of the coolant of high temperature and-high pressure for engine cooling water is arranged between the compressor and the outdoor heat exchanger in order to bring about the similar effect as the outdoor heat exchanger being improved in radiation efficiency. Consequently, even if the radiation efficiency in a radiator is small, it is possible to assure the cooling performance of an evaporator.
Here noted when an engine starts, the temperature of lubricating oil inside the engine is relatively low together with the temperature of the engine. Then, friction of sliding parts of the engine is increased to reduce the efficiency of the engine. In the above coolant cycle, nevertheless, since the heating of cooling water by the coolant of high temperature allows the friction to be reduced in an early stage, the warm-up time for the engine can be shortened to improve its fuel consumption. Correspondingly, since the water temperature is also elevated in an early stage, the instantaneous effect at heating period can be improved.
In an air conditioning apparatus employing the above coolant cycle, however, the heat exchange between the coolant and the engine cooling water is always carried out in the water-coolant heat exchanger. If the water temperature of the engine cooling water is remarkably high, there arises a possibility that the water temperature is further elevated as a result of heat-exchanging at the water-coolant heat exchanger, so that the sliding parts of the engine may be damaged (e.g. burn-in) because of insufficient cool-down of the engine.